1. Field of the Invention
The present invention pertains to a high-pressure fluid seal and in particular one incorporating a rotating mechanical seal which is used as the shaft sealing device in compressors for air conditioning systems and in particular for automotive air conditioning systems which use carbon dioxide as the refrigerant.
2. Description of Related Art
Automative air conditioning units have traditionally used fluorocarbons as the refrigerant. Adverse effects of the fluorocarbons (Flon R 134a) on global warming have, however, prompted studies with a number of other refrigerants in order to replace fluorocarbons. One of the most promising refrigerants for this purpose is carbon dioxide (CO2).
While the use of fluorocarbons as a refrigerant requires, at normal conditions, intake pressure of about 0.2 MPa at the compressor and displacement pressure of about 2 MPa, an increase in the required compressor pressure occurs when using carbon dioxide as the refrigerant up to intake pressure of 3 MPa and displacement pressure of about 7 MPa.
Described in U.S. Pat. No. 4,623,153 is a compressor which utilizes a fluorocarbon as the refrigerant and employs an oil-type shaft seal comprising a combination of a rubber ring and a synthetic resin ring. The use of carbon dioxide as the refrigerant, however, entails working pressure at the compressor which is higher than that indicated above regardless of the use of the oil-type seal.
Among the potential replacement seals is the conventional rotating mechanical seal which, at higher pressure, may be employed in tandem and when installed in a series of two each will reduce the loading on each seal to about the half. This approach is, however, deficient to the extent that the space required for the seals is too large.
In general, the sliding ring is manufactured from a slip-promoting carbon material and the backing ring from a slip-promoting material of greater hardness than the carbon slip-promoting material such as, for example, silicon carbide or tungsten carbide.
In rotating mechanical seals, the carbon slip-promoting material, which exhibits a lower modulus of elasticity, is deformed under pressure when it is subjected to high pressure due to the carbon dioxide pressure whereby damage to the sealing surfaces will occur. To avoid this problem, it is conceivable to use a combination of a hard material with a high modulus of elasticity such as silicon carbide or tungsten carbide for both parts in order to avoid deformation and damage to the sealing surfaces.
The sliding properties between a combination of these hard materials is, however, disadvantageous to the extent that under poor lubricating conditions, erosion or roughening of the surface or similar effects occur wherein the sealing performance is reduced.